The invention provides for a method for removing contaminants such as nitrogen oxides (NOx), particulates, and acid gases such as HCl and HF from gas streams arising from pickling operations in cleaning metallurgical components or sheets.
Mixed acid pickling is an important step in making stainless steel sheets by removing imperfections and contamination on the surface. Prior to pickling, sheets are subjected to series of operations for surface preparation. Following the pickling step, sheets are subjected to a passivation step.
The surface preparation steps involve subjecting sheets to hot molten alkaline salts, washing with detergents and treating in mixed acid bath containing sulphuric acid. These operations remove and dislodge impurities and some imperfections.
Pickling of stainless steel consists of passing the material through a highly oxidizing chemical bath of nitric acid (HNO3) and hydrofluoric acid (HF). Pickling aggressively removes any oxidized scale on the surface and prepares a smooth and continuous chromium oxide film.
Pickling is followed by passivation, which removes any free iron contaminants from the surface and develops a passive oxide film of chromium dioxide (CrO2) rapidly on the surface which prevents further oxidation sites to initiate rust and scaling.
In pickling of metals, especially austenitic and ferrites steels, where mixture of nitric acid and hydrofluoric acid, pickling operations deplete nitric acid in the bath generating significant quantities of nitrogen oxides. Due to the elevated temperatures (>40° C.) required in pickling operations, there are substantial amounts of vapors of pickling acids that arise over the pickling bath. Nitrogen oxides and vapors of pickling acids are collected by hood over the pickling baths and conveyed by a duct to a scrubbing system. In case of pickling stainless steel sheets, metal strips are continuously fed to the pickling bath through a narrow opening between the bath and the hood where some air also slips or leaks into the hood. Wet scrubbing operations remove acid vapors and a small fraction of nitrogen oxides from the gaseous exhaust stream. Nitrogen oxides are sparingly soluble gases and only slightly reactive, therefore it is difficult to effectively remove nitrogen oxides by industrially employed aqueous scrubbing systems.
Many technologies are used to reduce nitrogen oxides emissions from pickling operations. The first category of these technologies deal with lowering nitrogen oxides formation in the pickling operations while the second category of technologies include scrubbing with various reagents for capturing and converting nitrogen oxides into soluble products.
The first set of technologies includes adding hydrogen peroxide, urea and bubbling or sparging of oxygen-containing gas stream such as air into the pickling bath. These techniques suppress nitrogen oxides generation in the pickling liquor of aqueous solution of mixed acids either by chemical oxidation or reduction. Adding hydrogen peroxide or urea into the pickling liquor not only alters the composition but also causes deviations in the quality and chemistry of pickling. Although reagent dosing technologies offer cheaper alternatives for nitrogen oxides control, they are not widely used due to an onerous analytical support requirement and poor control of pickling quality. Bubbling or sparging of an oxygen-containing gas is not very effective when used as the only treatment.
When nitric acid in the pickling solution depletes, it decomposes. The chemistry of this transformation of nitric acid into nitrogen oxides is somewhat complicated. Nitric acid decomposes into oxyacids such as nitrous acid and finally into oxides of nitrogen commonly referred to as NOX. Nitrogen oxides being sparingly soluble, they are continuously released from pickling liquor during pickling operations to the gaseous phase residing over the pickling bath.
Adding reagents such as hydrogen peroxide or urea alters the chemical composition of the pickling liquor and suppresses NOx. Continuously and consistently characterizing the chemistry of the pickling liquor and monitoring the decomposition of nitric acid is challenging and difficult. Therefore, adding reagents to effectively suppress nitrogen oxides formation while maintaining the chemistry of pickling in real time is difficult. Any deviation affects the quality of pickling. Although these technologies offer lowering nitrogen oxides from pickling operations, they have limited success in pickling of the final product as it requires greater support from analytical chemistry to stay within the desired quality limits of pickling chemistry.
Some early patent literature also claims the use of compressed air sparging in the pickling bath along with use of hydrogen peroxide to lower NOx emissions. Oxygen in the air has very low solubility in the pickling liquor and therefore sparging air in the bath does not significantly lower NOx emissions. The present day practice is to use pickling tubs which have even lower liquid height than a classical pickling bath making air sparging even less effective. Additionally, it is known in the commercial manufacture of nitric acid, sparging air in the bleacher section enhances decomposition of oxyacids while converting only a very small portion of oxyacids to nitric acid which further corroborates limited effectiveness of sparging air (oxygen containing gas) directly into pickling bath.
The conventional nitrogen oxides control technologies such as SNCR (selective non catalytic reduction) and SCR (selective catalytic reduction) are also used in treating exhaust gas. Both SNCR and SCR require heating the gas stream from ambient to higher temperatures. SCR in particular is vulnerable to catalyst poisoning due to the occasional presence of acid gases such as HF when the acid gas scrubber malfunctions. Another approach is to use non selective reduction techniques using methane or natural gas. The energy cost in both selective and non selective methods is considerable though.
Pickling bath traditionally operates in the temperature range of 40 to 60° C. In order to implement SNCR or SCR, the exhaust gas stream from the pickling bath must be first scrubbed to remove halogen acid gas fumes (HCl or HF) and heated to the required temperature before subjecting it to SNCR or SCR technique. SNCR requires much higher temperatures and expensive capital equipment to implement. SCR is also equally expensive in terms of capital equipment but requires heating exhaust gas to a moderate temperature. The SCR catalyst is also prone to poisoning should halogen containing acid gas escape the wet scrubber. SCR is used with less than moderate success.
Wet chemical scrubbing is used with more moderate success. One or more chemical reagents are widely used to control NOx emissions. Reagents used are H2O2, caustic, sodium hydrosulphite, sodium chlorite and sodium sulphide in combination with caustic or alkali. Each one has its own set of limitations but in common, they require large size wet scrubbing apparatus, demand operational oversight to maintain performance, are costly in terms of reagents and produce huge quantity of aqueous waste that require elaborate processing in an effluent treatment facility. Most of these technologies are based on oxidizing or reducing NOx dissolved in the reagent medium in the wet scrubber. As mentioned earlier nitrogen oxides, consisting mainly of NO and NO2, are sparingly soluble gases and therefore require scrubbers substantially large in size to dissolve even in the medium containing chemical reagents. Jethani et al. (1990) have reviewed chemical reagents used in wet scrubbing of NOx.
The use of ozone for oxidizing nitrogen oxides is described in U.S. Pat. Nos. 5,206,002; 6,162,409; 6,649,132; and 7,303,735. The methods described in these patents are useful for nitrogen oxides oxidation, absorption in nitric acid manufacture, mixed acid recovery and techniques for oxidizing nitrogen oxides with ozone. However, they are not as well suited for scrubbing nitrogen oxides in pickling operations due to their being relatively cost prohibitive due in part to the cost associated in producing large amounts of ozone.
Ozone based low temperature oxidation processes are based on the chemistry of nitrogen oxides reaction with ozone that forms higher oxides of nitrogen. Solubility of NOx increases considerably with oxidation and the pentavalent form is easily and almost completely removed by wet scrubbing. The stoichiometric amount of ozone required to convert one mole of NOx (in the form of NO) to a pentavalent form is about 1.5 moles of ozone and 0.5 moles if NOx is in the form of NO2.
Ozone is an unstable gas and is generated on-site and on-demand using gaseous oxygen. The ozone generation is modulated rapidly based on amount of NOx present in the exhaust gas stream. Ozone generation is done in a well engineered system consisting of an ozone generating vessel and power supply unit. Ozone is produced by flowing an O2 containing gas stream through a corona caused by electric discharges. Ozone in a high concentration can undergo rapid decomposition even leading to explosion. Most commercially available generators provide 8 to 12 wt % conversion of oxygen to ozone. For making 1 kg of ozone with current state of art technologies, 10 to 12 KW/Hr of power is required to obtain 10 wt % conversions.
Various ozone based methods described in the aforementioned patents are very robust and extremely effective in achieving ultra low levels of NOx. A typical exhaust from pickling bath has NOx contents in the range of 1000 to 4000 PPM by volume. In order to cause effective removal 1.5 moles of ozone per every mole of NO and 0.5 moles per every mole of NO2 is required. The typical NO to NO2 ratio in pickling is 60:40 requiring an addition of ozone that would be equivalent to 0.44 volume % of the exhaust gas flow. This is way too much ozone and turns out to be a huge quantity for an average production scale facility. Ozone generation is expensive both in capital and operating costs. On-site generation requires large sum of fixed capital. Operating costs include heavy consumption of power and oxygen. Due to these very high operating and capital costs, the use is limited. Any attempt to reduce this ozone requirement can make ozone oxidation approach commercially attractive.
The methods described by this invention recover part of the nitric acid by regenerating it in situ within the pickling operations thereby lowering concentration of NOx in the exhaust (effluent) stream leaving the pickling bath. As per this invention the NOx leaving the bath are substantially in the form of NO2 which requires only one third the amount of ozone to react compared to NO thus requiring significantly lower amounts of ozone. The invention also offers an option of using single wet scrubber eliminating need of an oxidation reactor or duct and the wet second scrubber.
Although various ozone based methods described in the aforementioned patents are very effective in achieving ultra low levels of nitrogen oxides emissions in the treated gas stream, they possess challenges with respect to economics and disposing of large amounts of effluent produced in removing nitrogen oxides. The method described for the present invention reduces the amount of ozone required, recovers part of the nitric acid by regenerating in situ and significantly reduces the amount of waste effluent generated.